The present invention relates to a displacement control mechanism for a variable displacement compressor. The displacement control mechanism supplies refrigerant from a discharge pressure zone to a control pressure chamber and delivers refrigerant from the control pressure chamber to a suction pressure zone, thereby controlling the pressure in the control pressure chamber, and changing the displacement of the compressor, accordingly.
In a variable displacement compressor having a control pressure chamber for accommodating a tiltable swash plate, the inclination angle of the swash plate is reduced as the pressure in the control pressure chamber is increased, and increased as the control chamber pressure is reduced. When the inclination angle of the swash plate is reduced, the stroke of the pistons is reduced, which decreases the displacement of the compressor. When the inclination angle of the swash plate is increased, the piston stroke is increased, which increases the displacement.
Japanese Laid-Open Patent Publication No. 2001-153044 discloses a displacement control valve that opens and closes a supply passage for supplying refrigerant from a discharge pressure zone to a crank chamber serving as a control pressure chamber. The displacement control valve includes a solenoid and a pressure sensing device that senses a pressure difference between two positions in the discharge pressure zone to actuate a valve body. When the refrigerant flow rate increases, the pressure difference between the two positions increases, accordingly. The pressure sensing device uses the increase in the pressure difference to displace the valve body in a direction opening a valve hole. This increases the pressure in the crank chamber, and thus reduces the displacement. In contrast, when the refrigerant flow rate decreases, the pressure difference between the two positions decreases, accordingly. The pressure sensing device uses the decrease in the pressure difference to displace the valve body in a direction closing the valve hole. This decreases the pressure in the crank chamber, and thus increases the displacement.
The displacement control valve includes a solenoid that applies electromagnetic force to the valve body against the pressure difference. The opening degree of the displacement control valve is varied by changing the value of a current supplied to the solenoid (duty ratio). The supplied current value (duty ratio) to the solenoid is determined by a controller. The controller determines the supplied current value (duty ratio) to the solenoid, for example, according to the difference between a set target compartment temperature and a detected compartment temperature.
If the variable displacement compressor operates with a shortage of refrigerant gas, the compartment temperature will never be lowered to the target temperature. To deal with the situation, the controller executes control for maximizing the supplied current value (duty ratio) to the solenoid (control for maximizing the inclination angle of the swash plate). That is, the variable displacement compressor operates with the maximum displacement even if the speed of the rotary shaft is high and the refrigerant flow rate is increased. Such high speed and large displacement operation applies a great load on the compressor, particularly on the swash plate, and is therefore unfavorable in terms of the reliability. Also, due to the shortage of the refrigerant gas, the discharge pressure cannot be increased. Therefore, high speed and large displacement operation with an insufficient amount of refrigerant gas causes the following disadvantages, for example, to a compressor having a hinge mechanism disclosed in Japanese Laid-Open Patent Publication No. 2004-108245. The hinge mechanism has a configuration in which a projection formed on the swash plate is simply held between a pair of projections formed on a rotary support to allow the swash plate to move freely along the axial direction of the rotary shaft. In the case of a compressor equipped with such a hinge mechanism, high speed and large displacement operation may cause inertial force of pistons to surpass the compression reaction force. As a result, the inclination angle of the swash plate in the maximum displacement operation can surpass a predetermined maximum inclination. If the inclination angle surpasses the predetermined maximum inclination angle, the pistons can collide with the plate in which suction valve flaps are formed.
Also, even if there is a sufficient amount of refrigerant gas, high speed and large displacement operation of a variable displacement compressor is not favorable in terms of the reliability. Further, even if there is a sufficient amount of refrigerant gas, a great inertial force of the pistons can cause the inclination angle of the swash plate to surpass the predetermined maximum inclination angle in the case of the hinge mechanism disclosed in Japanese Laid-Open Patent Publication No. 2004-108245.